1. Field
The invention relates to electronic tape timers for video tape recorder/reproducers (VTRs), and particularly to a correction circuit for providing frame rate pulses to a tape timer circuit of a quadruplex VTR, which pulses are a precise indication of the tape longitudinal position and movement relative to the head means, over the entire speed range of the VTR.
2. Prior Art
Tape timers are incorporated in video transport systems to accurately locate desired positions on the tape, and to continuously indicate the amount of relative elapsed time which has passed after starting the tape. The indications of tape position may be a measure of the time which has passed following the start of the tape transport at a tape reference point, or of the time remaining before a reference point is reached. To provide editing accuracies of the order of a frame video, very precise measurement of the longitudinal position of the tape relative to the record and reproduce magnetic head locations must be made.
To this end, various tape timer and corrector circuits are available in the art, wherein the components thereof are generally determined by the characteristics of the tape transport, and the degree of accuracy required by the particular application.
A basic tape timer utilizes a tachometer wheel coupled to rotate as the tape is transported thereover. Tachometer sensors of selected design provide pulses that are counted to provide information indicative of the length of tape which has passed the magnetic head location. The tach information is accordingly mechanically derived and its accuracy therefore depends upon maintaining a constant relationship between the number of tach pulses generated and the actual longitudinal length of tape transported past the magnetic head location. Tape slippage, creep, changes in tape stretching and transport tension, the fact that the tape is reproduced on different recorders than it was recorded on with differences in tension, capstan diameters and capstan surface slip coefficients, etc., causes changes in the relationship which, in turn, causes inaccuracies in the length of tape measured. Such mechanical tachometers have the advantage of providing continuous tach information without interruption.
A further tape timer system utilizes timing information recorded on the tape itself, which information is read from the tape as it passes the magnetic head location. Such a system circumvents the inaccuracies of the mechanical tachometer system since such recorded information accurately reflects the length of tape transported past the magnetic head locations. In video recorders, of the quadruplex type, a control track made up of a given standard of components, is recorded longitudinally along the tape length. For example, such a composite control track has two components; a sinusoidal component at the head rotational rate, and a frame pulse at the television frame rate, as directed by NTSC, PAL or SECAM formats. The control track pulse is thereafter reproduced as, for example, during the playback and or editing process, to provide accurate tape timing information at the frame rate.
Still further tape timers in the art provide a combination of the mechanical tachometer and control track systems of previous mention, to gain the advantages of both while circumventing the disadvantages of either. In this latter, more accurate system, the mechanical tachometer is generally employed as the principle source of tape timing information, while the control track pulse is employed to correct any deviations of the mechanical tachometer due to the inaccuracies caused by tape stretchage, creep, tension variations, differences between recorders, etc. To this end, some form of phase comparison is made between the information from the tachometer and the control track, which allows the tape timer system to correct the tachometer signals in response to the control track information.
The latter, more sophisticated system, is preferable for use in video recorder/reproducer systems when editing or splicing of information is to be performed, particularly when attempting to locate an edit to within accuracies of one frame of video. Heretofore, however, separate and expensive equipment have been required to provide editing to such accuracies, particularly in quadruplex video recorders, since the control track recorded on quadruplex systems is formed of multi-frequency signals and is thus difficult to extract precisely over the full range of operating speeds. For example, unlike some video recorders, the quadruplex VTR provides a standard control track which includes an unbiased 240 Hz signal recorded close to the saturation point of the tape thereby giving rise to severe third harmonic distortions of the order of 30 percent. In addition, the control track includes spaced frame pulses the period of which is dependent upon whether the system is employing the NTSC, PAL or SECAM recording format. It is very difficult to detect and extract with precise accuracy such a complex control track signal over the range of tape speeds encountered when shuttling tape rapidly from one position to another.